The objective of the proposed research is to investigate changes in level and ultrastructural location of several elements associated with biological mineralization. Bone cells and matrix in early stages of bone formation will be studied. Avian shell gland epithelium in the act of forming an egg shell will be compared with the resting shell gland. Methodology will rely on ultracryotomy, freeze-substitution fluids which contain ion trapping agents and low-temperature ashing for maximum preservation of mineral constituents. Electron microprobe analysis will be employed to identify elements and to demonstrate changes in their levels. Particular attention will be paid to subcellular organelles.